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If they haven't put an impermeable membrane there, it's going to leach through - the efflorescent bloom is a give-away too.
You might be better off having a stud there, or insulated plasterboard. You lose a few inches of room, but you gain a lot of heat retention, and don;t have to worry about 100+ years of nasty shit discolouring your walls.
(You may have to think about how you ventilate though - I'm assuming it's an exterior wall, so condensation may be an issue if you have a stud / plasterboard against it)
It's completely dry to touch. Been like that for ~2 months now. Had dehumidifier running pretty regularly since it's been done and the patch hasn't reduced in size after initial drying out period.
Edit: This is what it was like when chimney breast was removed, hence thinking soot has soaked through plaster